Explain ${#arrayname[@]} syntax for array length in bash?

Question

I know that one can get the length of an array in bash by doing ${#arrayname[@]}.

My question is: is this just something that I have to memorize, or can this syntax be broken down into understandable parts? For instance, what does the @ symbol mean where one would expect to find the index? Why the #?

Answer 1

# at the beginning of a variable reference means to get the length of the variable's value. For a normal variable this means its length in characters. # is the "number" sign, so you can remember this as meaning "the number of things in the variable".

@ or * in an array index means to use the whole array, not a specific element, and instead of returning the number of characters, it returns the number of array elements. * is used as a wildcard in many contexts, so this should be easy to remember. Also, $* and $@ are used to mean all the arguments to a shell script, so the parallel with all the array elements should be obvious.

You can't just write ${#arrayname} because when you use an array variable without a subscript, it's equivalent to element 0 of the array. So ${#arrayname} is the same as ${#arrayname[0]}, which is the number of characters in the first element of the array.

Answer 2

You should memorize. :) The # usually means number. e.g. the

• $# - is the number of arguments
• ${#str} - length of the string $str
• ${#arr[@]}" - length (number of elements) of the array arr
• ${#arr} - the length of the 1st element of the array (like the str above)

Unfortunately the ${parameter#word} or ${parameter##word} has nothing with numbers. (it removes the shortest/longest word from the beginning of the parameter.

And also, the # .... is comment ;)